Steam generating unit with gas recirculation means for preventing air heater corrosion



Jan. 3, 1961 Filed Jan. 23, 1956 C. S. SMITH STEAM GENERATING UNIT WITH GAS RECIRCULATION MEANS FOR PREVENTING AIR HEATER CORROSION 3 Sheets-Sheet l 1 102 INVENTOR. Charles S. Smiih ATTOR EY Jan. 3, 1961 Filed Jan. 25, 1956 s SMITH 2,966,897

C- STEAM GENERATING UNIT WITH GAS RECIRCULA'I'ION MEANS FOR PREVENTING AIR HEATER CORROSION 3 Sheets-Sheet 2 FIG 2 ATT RNEY Jan. 3, 1961 5 SM TH 2,966,897

C. l STEAM GENERATING UNIT WITH GAS RECIRCULA'I'ION MEANS FOR PREVENTING AIR HEATER CORROSION Filed Jan. 25, 1956 5 Sheets-Sheet 3 H H 77 j 76 I: 83 :lti]

40 i1 40 Q Q Q E FIG 3 INVENTOR.

Charles S. Smhh BY K i QTFORNEY United States Patent STEAM GENERATING UNIT WITH GAS RECIRCU- LATION MEANS FOR PREVENTING AIR HEAT- ER CORROSION Charles S. Smith, Westfield, N.J., assignor to The Babcock & Wilcox Company, New York, N.Y., a corporation of New Jersey Filed Jan. 23, 1956, Ser. No. 560,489

6 Claims. (Cl. 122-479) This invention relates to a method of, and apparatus for, controlling the final temperature of superheated high pressure steam so as to maintain it at a predetermined value over a wide load range, the invention involving the use of gas recirculation in such a manner that corrosion of air heater metal is prevented. The method is carried out in a gas recirculating fuel fired steam generating and steam heating unit which, preferably, includes a gas pass leading from a fuel fired furnace and having a steam heater (i.e. a super-heater), a convection economizer and an air heater located in that sequence in the direction of flow of the furnace gases through the gas pass. For the purpose of maintaining a predetermined or optimum high superheated steam temperature over a wide load range, and for the purpose of attaining optimum steam generator efiiciency, the invention involves, over part of the load range, the withdrawal of gases from the gass pass at a position between the economizer and the air heater and the introducing of the withdrawn gases into the furnace. Such withdrawal of gases is, predominantly if not wholly, effected at the position indicated in the pertinent part of the load range. In the lowest part of the load range, and when several factors including decreased gas temperature and decreased gas flow contribute toward an undesirably low gas temperature in the air heater, and particularly near the gas exit portion of the air heater, the gas temperature is apt to be so low that a corrosive chemical such as H 50 or H 80 will be formed on the air heater tubes, subsequently causing such damage thereto as to necessitate outage of the steam generating unit for the purpose of repair or replacement of the air heater parts. The invention prevents or minimizes such a contingency by at least reducing, if not entirely eliminating the withdrawal of recirculated gases from a position ahead of the air heater at such lowest loads and providing for the withdrawal of the recycled gases at a position adjacent the gas exit of the air heater at such loads. At intermediate loads, a proportion of the withdrawn gases may be taken from a position ahead of the air heater and the remainder of the withdrawn gases taken from a position adjacent the gas exit of the air heater and the gases from these different positions mixed before their introduction into the furnace.

Preferably the invention involves automatic control means whereby a temperature responsive element disposed in the gas stream downstream of the air heater determines the position at which the gases are withdrawn and determines the proportion of withdrawn gases withdrawn at the position downstream of the air heater in a gas flow sense. The effect of such a temperature responsive element in determining the gas withdrawal may also be affected or modified by the control devices which provide outgoing control representations of load or the rate of steam generation, the rate of fuel firing, and the rate of air fiow through the air heater. In addition, the illusstrative steam generating and superheating unit involves appropriate control devices for maintaining a predeter-- 2,966,897 Patented Jan. 3, 1961 mined or optimum steam pressure in the unit, for maintainlng such a rate of steam generation that steam demand is fully met, and for maintaining the temperature of the superheated steam at a predetermined value over a wide load range, at least some of such control devices varying the rate of flow of withdrawn and recycled gases as the load varies.

The illustrative steam generating and superheating unit of the invention involves a gas recirculation system in which the flow of recirculated gases is increased as the load decreases, and thereby has a double effect in the control of superheated steam temperature, in relation to load. This double efiect involves a decrease in the rate of steam generation in steam generating furnace wall tubes as the flow of the recirculated gas is increased and, simultaneously, involves an increase in total mass flow of the gases over the convection superheater, as the rate of vapor generation decreases. The gas recirculation system for the illustrative unit involves a gas recirculating fan, fan outlet duct-work directing and distributing the recirculated gases along one or more furnace walls with its steam generating wall tubes so that the lower temperature recirculated gases are interposed between those wall tubes and the higher temperature unrecirculated gases, and fan inlet duct-work having two branches with one branch leading from the gas pass of the convection section at a position adjacent the gas inlet of the air heater and another branch leading from a position adjacent the gas outlet of the air heater. The appropriate or pertinent control devices include a gas flow regulator in each of these branches for the purposes above indicated. In one modification of the apparatus of the invention, these two branches unite or join before reaching the inlet of the gas recirculating fan, thereby providing for the mixing of two gas streams of different temperatures before the circulated gases are introduced into the furnace.

The furnace of the illustrative unit, with its steam generating wall tubes provides a zone of radiant heat transmission from high temperature gases to enclosed streams of a vaporizable liquid, and considering this subject matter, the invention may be said to involve a method of maintaining a predetermined temperature of superheated vapor under substantial pressure, and simultaneously preventing air heater corrosion, the method involving the generation of vapor at a substantial pressure by the radiant transmission of heat from a high temperature heating gas zone to enclosed streams of a vaporizable liquid, superheating the generated vapor by the transmission of heat from the gases from the radiant heat transmission zone, burning fuel to supply high temperature gases of the radiant heating zone, convectionally transmitting heat from the gases through metallic air heater components to combustion supporting air after the loss of heat from the gases in said superheating, supplying the heated combustion supporting air to the fuel burning zone, and simultaneously preventing corrosion of said components and maintaining a predetermined superheated vapor temperature over a wide range of rate of vapor generation by simultaneously withdrawing a regulated proportion of the gases after loss of heat therefrom in the superheating and introducing the withdrawn gases into the combustion zone or radiant heating zone, a controlled proportion of the withdrawn gases being taken from the gas stream before gas contact with said components and a regulated proportion of the withdrawn gases being taken from the gas stream after loss of heat therefrom to said components.

The invention will be concisely set forth in the appended claims but for a complete understanding of the invention recourse should be had to the accompanying description which refers to preferred modifications or 3 embodiments of the invention,-one of which is shown in the accompanying drawings.

In the drawings:

Fig. l is a vertical section or side sectional'view ofa steam generating and superheating unit involving the invention:

Fig. 2 is a diagrammatic front elevation taken from the left-hand side of the unit as shown in Fig.-l,'Fig. 2 showing the division wall separating the two furnace sections; and

Fig. 3 is a diagrammatic plan of the Fig. 1 unit.

Referring to the drawings, there is indicated a steam generating and superheating unit of the water tube type. As'indicated in Fig. 2, there are two furnace sections Hand 12 separated by a division wall 14, preferably including steam generating wall tubes appropriately connected into the fluid circulation of the unit. Such connections include the lower headers 15 and 16 to which the lower ends of the steam generating tubes of the division wall are connected, thelower portions 17 and 18 of these tubes being disposed as indicated in Fig. 2 to form sides of the hopper bottoms 19 and 20. The headers 15 and 16 are connected by appropriate circulatory connections to the water space of the steam and water drum 21. The upper ends of the tubes of the division wall 14 are connected to the steam and water mixture receiving chain ber of the drum 21 by appropriate circulatory connections including the division wall header 22.

The walls of the furnace chambers 10 and 12 opposite the division wall 14 similarly include steam generating wall tubes 23 and 24, connected into the circulation of the unit through appropriate circulatory connections including the lower headers 25 and 26 and the upper headers 27 and 28. The lower parts of the wall tubes 23 and 24 delineate the remaining inclined walls 29 and 30-of the hopper bottoms 19 and 20.

The side sectional elevation of Fig. 1 shows the furnace chamber 10 in vertical section, looking toward the furnace side wall steam generating tubes 23. The sidewall associated with the tubes 23 extends from thefront furnace wall 31 to the rear furnacewall 32, Fig. 2 showing the lower front wall header33 which, together with appropriate circulatory connections to the Water space of the drum 21 supplies water to the front wall steam generating tubes 34. These tubes are connected at their upper ends to an upper front wall header-35 from'which roof tubes '36 extend to the steam and watermixture receiving chamber of the drum 21.

The steam generating tubes 32 along the rear wall of the furnace chamber 10 receive water through the lower header 37 which is connected by downcomers 38 shown as'leading vertically upwardly along the rear wall 39 of the unit to the water space of the drum 21.

Each furnace section, as shown in the drawings, is fired byfuel burners 40 which project fuel and air streams downwardly into the furnace between adjacent roof tubes 36', the fuel and primary air tubes of the burners being disposed in a windbox 41 receiving heated air from the air heater generally indicated at 42. This heated air is conducted-from the air outlet of the air heater by the ductwork including the duct components ES-45.

The combustion gases pass downwardly through each furnace section and turn to the right to enter the gas 2 turning space 46 at the lower end of an upfiow gas pass 47 between the rear furnace wall and the rear wall 39: ofthe unit. The floor or inclined wall 43 of the gas turning space 46 includes inclined sections of the tubes 32 with succeeding screen sections 49 and 50 of these tubes widely spaced to provide for appropriate gas flow into the gas pass 47 and also to provide a radiant heat screen for the secondary superheater section 51 disposed in the gas pass adjacent the screen tube sections. Beyond the secondary superheater section 51, the gases flow across the spaced tubes of a primary convection superheater 52 which receives steam through the inlet header?" 53 and the-superheater supply tubes 54, the latter-deading from the steam exit space of the drum 21.

After leaving the primarysuperheater 52, the gases enter the gas turning space 55 at the top of the gas pass 47 and then pass over the eoonomizer 100 and through the breeching 56. Thence they pass through the horizontally spaced tubes 57- been airheater42. Thelatterfi is'preferably divided into two sections with the tubes-57 terminating in a gas chamber 58 from which the shorter air heater tubes 59. conduct the gases to a breeching .orflue 60.

Air to be heated for supply to the burners 40- enters the duct work 61 and passes to the right between and over the spaced tubes 59 0f the lower section of the air heater. Thence the"gases'turn'upwardly in the air chamber 62 and are directed reversely by the bafile 63 through the second air pass 64. In this pass the air is heated by its passage between and over the gas conducting tubes of the air heater and upon emerging from'this seconds pass'the gases turn in theair chamber 65 to pass across: the tubular sections of the third air pass 66 of the air." heater. They are caused to' flow-in this manner by'thev. bafile 67 which terminates in the air turning chamber. 68..' From the upper part of this chamber the gases are caused,. by the baffle 69, to pass to the right through the fourth..- air pass 70 of the air heater and thence'into the air turn ing chamber 71. From this chamber the gases pass'to: the right acrossand over the parts of the tubes 57 in the fifth air pass 72. Thence the heated air passes through. the duct-work 43-45 to the windbox 41 and then be. tween the successive roof tubes 36 into the furnace'cham ber.

As a part of the superhea't steam temperature. control:.. of" the steam'generating unit, the latter includes a superel heated steam 'attemperator 73' interposed relative tothc. outlet header 74 of' the primary superheater 52 and theiinlet'header 75 of the secondary superheater' 5i. atternperator maybe of'the type indicated-in the patent." to Fletcher et a1. 2,550,683, of May 1, 1951.

The steam temperature control of the unit further eludes gasrecirculation means to correct, or'compensaie for inherent tendencies of a steam gene-rating unitof the" type here disclosed. These tendencies, include a tendency: for the superheated steam temperature to drop below a predeterminedor optimum value as load, and the rate of fuelfiring is reduced 'in accordance with representa tions of steam demand. For example, if the unit operates. at a control point load of 500,000 lbs. of steam per hour." at a pressure of 2000 psi. and at a final steam temperature of 1050 F. with no gas recirculation, then when the load drops to 250,000 'lbs. of steam per hounthesuper heat temperature would drop substantially below the optimum temperature. Also the rate of steam generation I in the furnace wall tubes, resulting from the transmission of heat from the burning fuel and combustion gases does not drop at the same rate. The rate is smaller. To compensate for these-conditions and to promote proper correlation the rate of steam generation and the attain-- ment of a predetermined superheat steam temperature over a wide load range the rate of flow of recirculated I gases to the furnace is increased as the load is reduced. This result'is attained by a gas recirculation system. and appropriate controls therefor, including devices for measuring superheat steam temperature, load, and possibly other operative influences and emitting pulse representations representative of these influences. The gas recirculation system' includes a gas recirculating fan '76 operated by an electric motor 77. Associated therewith is fan :.i outlet duct work-including the rnain duct 78 andtther branch ducts 79 and 80 having therein the gas flow regulators 81 and 82. The branch duct Eli-leads to a cross duct 83 the outlet of which directs recirculated gases l let of which directs the gases downwardly along the front wall 31 and its wall tubes 34. Such downward direction of recirculated gases in a stratum along a furnace wall operates to interpose relative to the adjacent steam generating tubes and the higher temperature unrecirculated gases a stratum of lower temperature recirculated gases. This action, as the flow of recirculated gases is increased, reduces the heat emitting circumference or volume of the unrecirculated gases, crowds the unrecirculated gases into a small space, and, when the recirculated gases are introduced along the front wall, decreases the residence time of the unrecirculated gases within the furnace chamber, all of these effects reducing the rate of heat absorption of the pertinent steam generating wall tubes.

Simultaneously with the above indicated effects the increased rate of flow of recirculated gases into the furnace increases the gas mass flow through the gas pass 47 and over the superheater. This action increases the final superheated steam temperature and, with appropriate controls changes that temperature to the predetermined, or optimum value. A primary operative control over the rate of gas recirculation may include a steam temperature responsive device disposed at 85 in the steam line leading from the secondary superheater outlet header 86. This temperature responsive device may transmit, by known control mechanisms, actions which appropriately change the r.p.m. of the recirculating gas fan 75 and/or the positions of the gas flow regulating dampers 81 and 82. This superheated steam temperature control of recirculated gas flow may be also appropriately modified by representations of load, or steam flow, in the line.

The gas flow recirculating system also includes recirculating gas fan inlet duct-work including a main duct 87 having a branch 89 leading to the breeching 56 and having therein a gas fiow regulating damper 88. Communicating with the main duct 87 is another branch duct 90 leading to the flue or breeching 60 at the gas outlet of the air heater 42 and provided with a gas flow regulating damper 91.

Long experience with high capacity units of the pertinent type have shown that corrosion of the gas conducting tubes of the air heater in the first air heater air pass 59 occurs and, after a substantial period of operation this corrosion may result in the necessity of shutting down the unit for the purpose of air heater repair which may include replacement of the corroded tubes. Such corrosion may result from a number of conditions which include the burning of a fuel which causes the existence of sulphur dioxide or sulphur trioxide and water vapor in the gases approaching the pertinent part of the air heater. Contributing influences may involve reduced rate of gas flow, reduced gas temperature and a reduced air temperature, such conditions tending to exist, predominantly, at the lowest loads. To compensate for the effect of such conditions and to minimize, if not prevent, the corrosion of air heater metal, the invention involves such regulation of the gas flow regulating dampers 88 and 91 that, at lowest loads a predominant part of the total recirculated gas flow if not the entire flow, takes place through the branch duct 90 leading from the flue or breeching 60 at the gas outlet of the air heater. Under such conditions the gas flue regulating damper 88 will be closed or nearly closed, and the regulation of the dampers 88 and 91 may be automatically effected from control actions initiated by a metal temperature responsive device 92, such as a thermo-couple preferably disposed on air heater tubes at a position adjacent the cold air entry of the air heater. Appropriate control components transmit representations of variations in air heater metal temperature, indicated by the device 92, into gas control pulses which will position the gas flow regulating dampers 88 and 91 in such a manner as to maintain the gas and air heater metal temperatures at the position 92 sufficiently high to prevent air heater metal corrosion. The

control influences emanating from the temperature responsive element at 92 may be also coordinated with indications or representations of steam flow at so that, ordinarily, maximum flow of recirculated gases through the branch duct takes place at the lower or lowest parts of the load range. The dampers 88 and 91 may be regulated by pneumatic or electric power devices and 101 actuated by control impulses from a controller 102 in response to indications of temperature obtained from element 92. The control impulses from controller 102 are modified by impulses obtained from device 85 positioned in the steam outlet line.

The direct effect of the invention, at the lowest parts of the load range is to raise the gas temperature of the air heater and thus raise the air heater metal temperature.

The above indicated extraction of recirculated gases from the gas outlet of the air heater has a detrimental effect upon the overall efficiency of the unit, and for that reason the main flow of recirculated gases at a normal load or control point load and at loads thereabove, takes place through the branch duct 89 so that the airheater is interposed as a heat absorbing agency between the ultimate gas outlet of the unit and the point at which the recirculated gases are withdrawn. Preferably a predominant part, if not all of the recirculated gases flow through the duct 89 in upper parts of the load range when the superheat control agencies call for gas recirculation.

At intermediate points in the entire load range and at points near lowest load, a portion of the total recirculated gases may flow through each of the ducts 89 and 90 with a controlled and predominant proportion of the flow of gases through the branch duct 90. This is effected by appropriate control mechanism activated or influenced by temperature responsive devices such as that disposed at 92, modified by other control influences such as representations of load or steam flow at 85.

What is claimed is:

1. In a method of controlling superheated steam temperature over a wide load range and simultaneously pre venting corrosion of air heater metal in a steam generating unit including steam generating furnace wall tubes, fuel burning means for the furnace, a steam superheater subject to the heat of the gases of the furnace, and a convection air heater subject to the flow of gases from the furnace after loss of heat from the gases in the superheating; the method comprising the withdrawal of a regulated proportion of the gases after loss of heat therefrom in superheating, introducing the withdrawn gases into the furnace in controlled amounts varying inversely with the rate of steam generation and thereby maintaining a predetermined superheated steam temperature over a wide load range, said withdrawal of gases taking place at least predominantly from a position in the gas flow path upstream of the air heater in an upper part of the load range,

and supplying a substantial proportion of the total withdrawn gases from a position adjacent the gas exit of the air heater over a lower part of the load range, and controlling the proportions of gas withdrawn before and after the air heater in response to the temperature of gases leaving said air heater to increase the gas flow withdrawn from adjacent the gas exit of the air heater as the gas temperature leaving the air heater decreases toward its dew point temperature. I

2. In the operation of a steam generating and superheating unit of the type having a gas heated furnace chamber the walls of which include vapor generating tubes, a superheater receiving the vapor generated by said tubes and including a convection section heated by the gases from the furnace chamber, a convection air heater receiving heat from the gases after loss of heat therefrom in the superheating, and a gas recirculation system receiving low temperature heating gases from a point in the gas flow beyond the superheater and delivering those gases to the furnace chamber for superheat control over a wide1 1oad:range;the method which comprises regulating the flow of recirculating gas in inverse relationship to the rate ofjsteam flow from said unit, extracting all of the recirculated gases from a position in the gas flow path ahead of they gas inlet to. the air heater in the upper part of the load range; extracting .in a lower part of the load range, asubstantial proportion of the total recirculated gases from a point in the gas flow path beyond the air heater to prevent air heater corrosion at low loads; and mixing together, in an intermediate part of the load range, the gases extracted from the .two different positions prior to the entry of the recirculated gases into the furnace chamber; andcontrolling the proportions of gas withdrawn before and after the air heater in response to the temperature of gases leaving said air heater, said gas withdrawal from after. the. air heater being increased when the temperature of gases leaving said air heater decreases towards its dew point temperature to avoid air heater corrosion.

3., In a steam generating unit, wall means including steam generating tubes defining a furnace, means for firing-the furnace, means forming a gas pass leadingfrom the gas outlet of the furnace, a superheater subject to heat of the gases flowing from the furnace through the gas pass for heating steam from said wall tubes, an economizer disposed within the gas pass beyond the superheater for heating feed-water for flow to the wall tubes, a convection air heater disposed within the gas pass downstream of the economizer, a gas recirculation system including a fan and branched fan inlet ductwork with one branch leading togas flow in the gas pass at a position between'the 'economizer and the air heater and the other branch leadingfrom the gas pass at a position downstream,

oflthe air heater in a gas flow sense, the gas recirculation system, also includingfan outlet ductwork leading to the furnace, temperature responsive means subject to the gas flow at a position adjacent the gas outlet of the air heater, separate recirculated gas flow regulation means disposed in each of said branches, control devices including damper operating devicesoperatively connecting said temperature responsive means and the gas flow regulators to increase the gas flow through the fan inlet ductwork branch adjacent the air heater outlet when the temperature of the air heater contacting gases falls below a predetermined value, and control means influenced by changes in the temperature of the superheated steam at the outlet of the superheater, for controlling the total ,fiOW of recirculated gases into the furnace in inverse relationship to change in said steam temperature.

4. In a steam generating unit Wall means defining a furnace having a gas outlet said means including steam generating wall tubes fuel burning means for firing the furnace means forming a gas pass leading from the gas outlet of the furnace a superheater subject to heat of the gasesflowing from the furnace through the gas pass, means vfor conducting steam from the wall tubes to the superheater, a convection air heater disposed within the gas pass downstream of the superheater, means connecting, the air outlet of the air heater to the fuel burning means, a gas recirculation system having a fan and fan inlet ductwork including one duct opening to said gas passat a position between the superheater and the air heater and another duct opening to said gas pass at a position downstream of the air heater in a gas fiow sense, the gas recirculation system also including fan outlet ductworkt leading to the furnace, and means changing the propon; tionate flow of the total withdrawn gases through saidf inlet ducts, said last named means including means r sponsive to the temperature of the heating gases at a pos tion adjacent the gas outlet of the air heater and opera tive to increase the gas flow through the downstream inlet; duct and decrease the gas flow through the upstream inlet ductwhen the gas temperature approaches a value at which air heater corrosion would take place.

5,.ln a steam generating unit, wall means defining a furnace having a gas outlet, said means including steam generating wall tubes, fuel burning means for firing the furnace, means forming a gas pass leading from the gas, outlet of the furnace, a superheater receiving the generated steam and subject to the heat of the furnace gases, means for conducting steam from the wall tubes to thesuperheater, an air heater disposed within the gas pass for heating combustion supporting air for delivery to thefurnace means connecting the air outlet of the air heater to the fuel burning means, a gas recirculation system including upstream and downstream inlet ducts leading from positions substantially spaced in the gas pass and in the direction of gas flow through the air heater, said gas recirculation system also including gas flow impelling' means and outlet ductwork leading therefrom to the furnace, and means responsive to heating gas temperature near thegas outlet of the air heater for controllably varyv ing the proportions of total recirculated gas flow through 1 the different inlet ducts, the last named means increasing the gas flow through the downstream inlet duct when the heating gas temperature nears a value at which air heater;- corrosion would take place.

6. Ina steam generating unit, Wall means defining-a furnace having a gas outlet, said means including steam; generatingwall tubes, fuelburning means for firing the furnace, means forming a gas pass leading from'the gas outlet ofthe furnace, a superheater-receiving the-generated steam and subject to the heat of the furnace gases, means for conducting steam from the wall tubes to the: superheater, an air heater disposed within-the gas-pass for heating combustion supporting air for deliveryrto the furnace, means connecting the air outlet of the air heater to the fuel burning means, a gas recirculation system: including upstream and downstream inlet ducts leading. from positions in the gas pass substantially spaced apart in the direction of gas flow through the air heater, said gas recirculation system also including gas flow irnpelling means and outlet ductwork leading therefrom to the-fun. nace separately from the heated air flow, said meansre sponsive to load change for. controllably varying-the proportions of total recirculated gas flowthrough the differ ent inlet ducts, said last named means increasinglthe gasflow through the downstream inlet duct and decreasing. the gas flow through the upstream inlet duct when, the;

load approaches a value at which air heater corrosion,

would take place. 1

References Qited in the file of this patent UNITED STATES PATENTS 2,730,999 Birkner Jan. 17, 1956,

2,837,066 Blaskowski June-3, 1958;

FOREIGN PATENTS 955,787 France July 4, 1949- 1,080,188 France May 26, 1954 

